The nation's first missile defense test in 1997 was flawed because a sensor could not distinguish between a warhead and decoys, congressional investigators said in a report released Monday. The Pentagon said the findings were outdated.

The Pentagon, contractors TRW and Boeing and a Massachusetts Institute of Technology review team all exaggerated the success of the test, said the General Accounting Office, Congress' investigative arm.

"If we can't tell the warhead apart from a decoy, what good is it?" asked Rep. Edward Markey, D-Mass., who released the report.

The sensor used technology similar to that in use today, according to Markey aides. The Pentagon said the current system combines the 1997 technology with another, totally different system for distinguishing warheads and decoys.

Air Force Lt. Col. Rick Lehner said the GAO report "doesn't matter, because that was a test of hardware that hasn't been part of the missile defense program in more than four years."

The 1997 test, which did not involve an attempt to intercept a warhead, tested a system made by Boeing and TRW that was rejected. The Pentagon accepted a competing system by Raytheon that relies on a different sensor that uses a different design and different means of discriminating warheads from decoys.

Lehner said the Pentagon agrees with an FBI investigation that there was a scientific disagreement, in which some experts believed the original reporting was adequate for the first test, and others disputed it.

The FBI reviewed the case after a fired TRW employee alleged in a lawsuit that TRW falsely reported or hid information to make the Pentagon believe the system worked.

TRW said the GAO report fails to support claims of fraud raised by former employee Nira Schwartz and concludes that TRW neither withheld nor manipulated data.

"The GAO's failure to find any instance of fraud on the part of TRW is not surprising, given that three other government agencies, including the most recent exoneration in May by the FBI, dismissed these same charges," TRW spokeswoman Marynoele Benson said.

Schwartz was employed by TRW for six months. During her tenure, TRW said she did not have the security clearance necessary for access to critical data relating to the company's work on the missile defense program.

The GAO did not offer conclusions on whether the test was a failure, but pointed out the flaws and questioned the favorable reviews.

The report found:

—The sensor cooling and calibration systems malfunctioned and it often detected targets where there were none. The contractors described the sensor performance as excellent.

—The contractors analyzed only about 12 seconds of data out of more than 60 seconds available, because of sensor malfunctions and other problems. Toward the end of the flight test, the software began to incorrectly identify a decoy as the warhead, but these date were eliminated from the analysis.

—An engineering team that reviewed the test did not verify whether the data were accurate or whether the software was able to discriminate between a warhead and a decoy. Despite this, the review was used by the Pentagon's Missile Defense Agency to dismiss allegations of fraud.

—Many problems and limitations of the flight test were not immediately disclosed by contractors and their claims that the system distinguished the warhead from the decoys have not been independently verified.